<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
    <script type="text/javascript">
        let num1 = 3
        var num2 = 5
        var result = num1 + num2
        console.log(result)
        var str = '10';
        console.log(num1 + str);//'310'
        console.log(num1 + parseInt(str));//13
        //alert(str);

        var age;
        console.log(age);
    </script>
</body>
</html>